The manufacture of polystyrene (PS) foam rigid packaging trays (absent a barrier layer) is well known. Such trays are used in grocery stores to protect and display fresh meat, poultry, cooked meat, and produce. The trays are commonly made by thermoforming polystyrene foam sheet.
Polystyrene trays are manufactured in a two-stage process covering (1) extrusion and (2) thermoforming. For the extrusion, pelletized polystyrene resin, "optional" scrap, citric acid and sodium bicarbonate are fed into an extruder at around 400.degree. F., melted, and mixed. Then, the hot mixture is blown with a hydrocarbon such as pentane and/or CO.sub.2 gas to foam the material. The tubular foamed PS sheet expands as it exits the extruder. The tube is slit into sheets, wound into rolls and taken to an outside storage yard to cure for about 3 days. This curing allows residual pentane to escape and is not required when 100% of CO.sub.2 is used to foam the material. Curing time is reduced when a hydrocarbon/CO.sub.2 blend is employed.
For the thermoforming, the roll of cured polystyrene foam sheet is fed through an oven to heat it near to its softening point. The hot sheet is then forced into tray molds by vacuum, air, and mechanical pressure. The molded foam sheet is then fed through a trim press where a punch and die mechanism cuts the individual rigid trays from the sheet. The trays are packaged in polyethylene bags and stored in plant warehouse space prior to shipment to customers.
Polystyrene foam material scrap is generated in both the extrusion and thermoforming operations. Extrusion scrap is mainly in foam sheet form and results from machine start-ups, and size and color changes. Thermoforming scrap is primarily trim scrap, the skeleton remaining after the trays are cut from the foam sheet, and trays rejected for quality control criteria. Reprocessing of scrap material is accomplished by first grinding the scrap into a "fluff" form and then extruding the fluff into pellet form, which is then usable as the "optional" scrap in the foam sheet extrusion process.
Furthermore, marketing and distribution practices in the sale of food increasingly require that the packaging materials provide better shelf life. Thus packaging materials with lower oxygen transmission rates are more and more desirable, particularly for oxygen-sensitive products such as fresh red meats or cooked meats.
Accordingly, packaging material with a low oxygen transmission rate has been provided by adhering a flexible barrier film to the extruded polystyrene foam sheet prior to thermoforming into trays so that the trays are barrier foam trays.
Foam trays having a flexible polymeric film sheet adhered thereto, are known from U.S. Pat. No. 4,847,148 (Jul. 11, 1989) to Schirmer; U.S. Pat. No. 3,748,218 (Jul. 24, 1973) to Newman et al; U.S. Pat. No. 4,832,775 (May 23, 1989) to Park et al; U.S. Pat. No. 3,793,135 (Feb. 19, 1974) to Monia; U.S. Pat. No. 4,055,672 (Oct. 25, 1977) to Hirsch et al; U.S. Pat. No. 4,076,570 (Feb. 28, 1978) to Medley et al; U.S. Pat. No. 4,111,349 (Sep. 5, 1978) to Buckler et al; U.S. Pat. No. 4,332,858 (Jun. 1, 1982) to Saitoh et al; U.S. Pat. No. 4,558,099 (May 13, 1986) to Diez; and U.S. Pat. No. 4,659,785 (Apr. 21, 1987) to Nagano et al. The disclosures of these references are incorporated herein by reference. It is also noted that various devices are known to enhance the microwavability of such trays for use in microwave heating of foods in microwave ovens, such as adding particulate polyphenylene oxide to the foaming polystyrene or perforating the polystyrene sheet. Typically in the past saran, also known as vinylidene chloride copolymer, was used for the flexible film for an oxygen barrier layer on such foam trays.
However, a problem arose in what to do with the barrier foam scrap, particularly the thermoforming trim scrap, i.e. the skeleton remaining after the trays are cut, when the flexible barrier film had saran in it. When saran-containing barrier foam scrap was ground into fluff, the fluff pelletized, and the pellets placed in the extruder with the virgin polystyrene pellets, citric acid and the sodium bicarbonate, it was found that barrier scrap burned in the extruder and so could not be used.
In today's world, the emphasis is great on recycling. However, the saran-containing scrap was unusable and had to be discarded when trays were cut. Skeleton scrap, i.e. the thermoforming trim, is typically 24 to 25% by weight, but can be as much as 30% by weight or more of the sheet when trays are cut. Thus, from an environmental/pollution standpoint, it is desirable that the scrap, particularly thermoforming trim, be reclaimed and recycled, not discarded.